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Hartley Oscillator

  1. Mar 8, 2007 #1
    Hello All,

    I was just reading an article on Wikipedia about Hartley Oscillators and they mention this at the bottom of the page:

    I'm assuming that the harmonics are coming from the transistor in the circuit? Could this be solved if a vacuum tube were used instead of a SS device?

    Thanks,
    Jason O
     
  2. jcsd
  3. Mar 8, 2007 #2
    Harmonics ALWAYS come from the active device in the circuit. I don't think that a tube will reduce the harmonics. Tubes typically have different harmonics than transistors do though. I don't recall which is which, but one has more energy in the odd harmonics and the other more in the even harmonics.
     
  4. Mar 8, 2007 #3
    In a hartley oscillator, the positive feedback loop runs through a coil voltage divider, as opposed to a capacitive feedback loop in a colpitts oscillator. Most inductors have a lower Q compared to a capacitor, as a result they are less selective and susceptible to a broad frequency response in a LC circuit.

    But the number one cause of harmonic content is the transistor itself. Different biases and configuration will result in different harmonics. For VHF, a Jfet transistor produces a much cleaner signal than BJT transistor. In either case, oscillators are usually followed by notch or low pass filters to kill unwanted frequencies.
     
  5. Mar 8, 2007 #4
    Thanks guys for the replies. I'm interested in building one of these for some experiments I'm doing. Now, concerning the harmonics, would they be as bad if the oscillator is running in the ULF to LF frequency range? Also, for the simplest versions of this oscillator (like the one pictured on Wikipedia, what kind of tuning range can one get with one of these?

    Thanks,
    Jason O
     
    Last edited: Mar 8, 2007
  6. Mar 8, 2007 #5
    Same thing for ULF (ultra low frequencies) would still have the same harmonics. But its impractical to build a Hartley oscillator for ULF as the coil would have to be a mile long.

    I wouldn't worry much about harmonics, the next closest harmonic would be down by 20 to 30 dB at least anyways.

    If you building the oscillator, I would recommend you build the Colpitts one. Because you only need one coil, and the capacitive feedback loop would be easy to tweak. Frequency tuning would have to be done by tuning the inductor though.

    As for the frequency range, the higher the frequency of operation the more frequency tuning leeway you will have, before the feedback loop dies.
     
  7. Mar 8, 2007 #6
    Hi, thanks for the tip. Would a Colpitts oscillator be practical for the frequency range that I am working with? Or is there some other oscillator scheme that is better suited for the frequency range I'm working with (100 Hz to 1 MHz roughly). I don't know much about them.
     
  8. Mar 8, 2007 #7
    Harmonics can be avoided if you couple the output of osc with a buffer-amp together with a passive filter circuit (with low-tolerance resistors and high order of the filter) Good decoupling and board design will give improved results.
     
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